1. Field of the Invention
The present invention is directed to apparatus and methods for incineration of toxic organic compounds. More particularly, the present invention provides safe and reliable incineration of toxic organic compounds present in low concentrations by injecting a clean fuel such as H.sub.2, CH.sub.4, or CO into the combustion effluents, or flue gas, to promote rapid oxidation of the toxic organic compounds.
2. Technology Review
Despite the use of apparently conservative design procedures, it is not uncommon for hazardous waste and other incinerators to fail to perform up to design specifications and government performance regulations. Part of this problem is due to the fact that incinerator design is highly dependent on laboratory simulations of the incineration process. There is theoretical evidence to suggest that currently acceptable laboratory simulations are highly misleading. Further, since the process of obtaining a permit to operate hazardous waste incinerators is indirectly dependent on these laboratory permitting procedures.
While there are a many different kinds of incinerators, incinerators are generally two stage devices. The first incinerator stage is a flame zone in which the bulk of the waste being incinerated is destroyed. Wastes, whatever their source, all tend to be quite variable fuels which can cause flame instability and other nonidealities. Because of these nonidealities, some of the waste can escape the flame zone.
The second stage is the section of the combustion chamber which is downstream of the flame. In this second stage any organic compounds which escaped the flame have an extended residence time at elevated temperature and may be destroyed by nonflame thermal oxidation. In most designs, the residence time is at least one second and the average temperature is at least 1000.degree. C. Since, however, 1000.degree. C. is only the average, some of the gases going through the thermal oxidation zone will experience temperatures considerably less than 1000.degree. C.
The procedures for obtaining a permit to operate hazardous waste incinerators are defined by the Resource Conservation and Recovery Act ("RCRA"). A permit to operate is issued after a trial burn has been executed or other appropriate test data obtained which demonstrate that the incinerator satisfactorily converts hazardous waste into nonhazardous compounds when operated under specified conditions. Satisfactory conversion is defined in terms of Destruction and Removal Efficiency ("DRE").
However, since most hazardous waste streams contain many compounds, a trial burn which involves the measurement of all of them would be prohibitively expensive. Intuitively, there is no need to measure every compound, because if those compounds which are most difficult to destroy by incineration are efficiently removed, it would be expected that the easily destroyed compounds are also removed. Consequently, the trial burn involves the measurement of a subset of compounds (the Principal Organic Hazardous Constituents, "POHCs") which are present in the input stream. If the DRE of these POHCs is 99.99 percent or greater, and certain other conditions met, then a permit to operate is granted.
This procedure assumes the existence of a list ranking individual compounds in terms of their relative ease of destruction under incinerator conditions. It can reasonably be argued that the flame zone is not compound selective. Organic materials escape the flame zone because of combustion instabilities and other factors which may be related to the overall properties of the fuel but which do not have any necessary relationship to the individual compounds in the fuel.
Thus, any difference in the incinerator's ability to destroy compounds in a mixture is a result of selectivity within the second stage of the incineration process. This would mean that in choosing the POHCs to be measured in a test burn the permit writer should specify those compounds in the waste stream which are the most resistant to nonflame thermal oxidation. Consequently, the most frequent view is that the incinerability of a given compound is its ranking in a list according to ease of nonflame thermal oxidation.
Numerous prior art studies of the nonflame thermal oxidation kinetics of pure organic compounds have been carried out. In many of these studies, the observed oxidation kinetics are used to estimate T99.99, the temperature at which a one second residence time is sufficient to produce 99.99% oxidation of the starting material. Table 1, quoted from Lee et al., "Revised Model for the Prediction of the Time-Temperature Requirements for Thermal Destruction of Dilute Organic Vapors," 75th Annual Meetino of APCA, New Orleans, La., June 1982, shows an example of this kind of data. From the values of T99.99, the least incinerable compound studied was methyl chloride, T99.99=1597.degree. F. (869.degree. C.), followed by methane, T99.99=1545.degree. F. (841.degree. C.), chlorobenzene, T99.99=1408.degree. F. (764.degree. C.), and benzene, T99.99=1351.degree. F. (733.degree. C.), and 17 other compounds with even lower values for T99.99.
TABLE 1 ______________________________________ T99.99 T99.99 Compound at 1 sec. (.degree.F.) at 2 sec. (.degree.F.) ______________________________________ Acrolein 1020 975 Acrylonitrile 1345 1297 Allyl Alcohol 1176 1077 Allyl Chloride 1276 1200 Benzene 1351 1322 Butene-1 1232 1195 Chlorobenzene 1408 1372 1-2 Dichloroethane 1216 1173 Ethane 1368 1328 Ethanol 1307 1256 Ethyl Acrylate 1132 1092 Ethylene 1328 1281 Ethyl Formate 1191 1145 Ethyl Mercaptan 778 704 Methane 1545 1486 Methyl Chloride 1597 1514 Methyl Ethyl Ketone 1290 1247 Propane 1330 1300 Propylene 1318 1247 Toluene 1340 1295 Triethylamine 1101 1058 Vinyl Acetate 1223 1164 Vinyl Chloride 1371 1332 ______________________________________
Based upon these data, the normal design for hazardous waste incinerators (a nonflame thermal oxidation zone of one second at 1000.degree. C.) would appear to be very conservative, and with incinerators being designed to operate at a much higher temperature than is necessary for 99.99% oxidation, it would seem unlikely that any should fail to meet regulations. The observed fact, however, is that even well designed incinerators do sometimes fail to meet specifications and regulations. One possible explanation why incinerators often fall short of performance expectations is that the laboratory measurements of incinerability substantially overestimate the ease with which organic materials in trace quantities can be oxidized.
The primary concern in many hazardous waste situations is to destroy a material that is initially at a concentration in the parts per million range and to achieve removals in excess of 99.99%. In the Lee et al. reference referred to above, the initial concentration of the organic compounds going into the reactor was 1000 ppm in all cases. No attempt was made to observe extents of removal greater than 99%. Similarly, in other studies of this type, the initial concentrations are orders of magnitude larger than might be found in practice, and the extents of reaction studied were much less than what is needed in practice. Thus, prior art laboratory studies of incineration generally involve initial and final concentrations which are orders of magnitude greater than those encountered in practice.
Recent evidence suggests the extent to which an organic compound is destroyed by incineration is not independent of the initial concentration of the compound, but that the fraction of the organic material which survives incineration increases with decreasing initial concentration of the organic material.
Research by Lyon and Hardy (Hardy et al., "Isothermal Quenching of the Oxidation of Wet CO," 39 Combustion and Flame 317-320 (1980); Lyon et al., "Influence of Inert Gas Pressure on the Kinetics of Wet CO Oxidation," 45 Combustion and Flame 209-212 (1982); and Lyon et al., "Oxidation Kinetics of Wet CO in Trace Concentrations," 61 Combustion and Flame 79-86 (1985)) suggest that the extent to which a compound is destroyed by incineration may be dependent on its concentration. In a study of CO oxidation it was found that for initial concentrations of CO of 2000 ppm and greater the reaction obeyed first order kinetics, i.e., for a given reaction time the percent of the initial CO remaining unoxidized was independent of the initial CO concentration. However, it was further found that below 2000 ppm the percent of the initial CO remaining after a given reaction time increased with decreasing initial CO concentration. This trend continued until a point was reached at which the final concentration of CO after reaction approached the initial concentration, suggesting that the reaction virtually stopped. For sufficiently small initial concentrations of CO, not merely was the oxidation rate smaller than one would expect from measurements at higher concentrations, but oxidation virtually did not occur.
The existence of a threshold concentration below which oxidation effectively does not occur may be explained by the branching chain theory of combustion reactions. Within the branching chain theory, the observed fact that combustion reactions are much faster than other types of reactions is explained as arising from two causes. First, combustion reactions are rapid because they are exothermic and self-accelerating. Combustion releases heat and the increasing temperature of a gas mixture undergoing combustion makes the combustion reaction go faster, a process which results in the formation of a flame. Of course, combustion can only happen for mixtures containing enough fuel to cause a large temperature change. This leads to the familiar concept of the flammability limit, wherein mixtures that do not contain enough fuel to be strongly self-heating would not burn.
The second reason combustion reactions are rapid relative to other types of reaction relates to the mechanism by which combustion occurs. Except for some special cases not relevant to this discussion, gas phase reactions occur by reaction mechanisms which involve reactive intermediates called free radicals, and the rate of the reaction is proportional to the concentration of intermediates. Most kinds of reactions are limited to whatever concentration of free radicals thermodynamic equilibrium provides.
Combustion reactions, however, occur by what is called a branching chain mechanism in which the reactions which oxidize the fuel produce free radicals. Increasing the concentration of free radicals increases the rate at which the fuel oxidizes which in turn increases the rate of free radical production. Thus, even at constant temperature, a combustion reaction is self-accelerating. One can have a mixture of fuel and air containing so little fuel that it cannot significantly self-heat. If this mixture is heated it will start to react slowly at first, but will react with an accelerating rate as the free radical concentration increases, achieving a rate that is literally explosive.
The foregoing are generally accepted combustion theory principles. The novel concept in Lyon and Hardy's work is the suggestion that there is a second threshold limit for combustion processes. The first limit, the flammability limit, arises from the fact that for a mixture to be self-heating it must contain enough fuel to cause a substantial temperature rise. The second threshold according to Lyon and Hardy, arises from the fact that generating a superequilibrium concentration of free radicals requires a minimum amount of fuel. The existence of a second threshold in combustion processes is supported empirically and by mathematical analysis and by computer modeling experiments. Computer modeling experiments have reproduced the experimental results.
While this second threshold is very low, too low to be a concern in most situations, it is a concern for incineration. In an incinerator the destruction of the toxic organic compounds should come extremely close to 100%. While part of this destruction is achieved in the flame, some of the toxic organic compounds inevitably escape the flame and must be destroyed by thermal oxidation in the combustion chamber.
Even though the second threshold for combustion is very low compared to the flammability limit, it may still be high relative to the emissions of toxic organic compounds which are environmentally acceptable. Thus, it is possible to have a situation in which the amounts of fuel, toxic organic compounds, and other materials escaping the flame are too small to sustain rapid reaction downstream of the flame but too large to be acceptable.
In view of the foregoing, it appears that current laboratory studies which provide the data for incinerator designs may give an overly optimistic description of incineration chemistry. Since, in practice, toxic organic compounds are often at very low concentrations, laboratory tests using concentrations significantly greater than normal may not be reliably used to predict the destruction for lower concentrations of the toxic organic compounds.
There is, of course, an obvious solution to this problem. By making the temperature sufficiently high, virtually complete destruction of the toxic organic compounds is possible. Toxic wastes, however, often contain a variety of toxic heavy metals. The temperature necessary to vaporize these toxic heavy metals varies with the metal. Some heavy metals, such as mercury, are volatile at modest temperatures, while others require very high temperatures for their vaporization. Thus, increasing the operating temperature of the incinerator can increase the number of different heavy metals which are vaporized and the extent of vaporization. This vaporization is irreversible because when the hot gases cool, the vaporized metals condense to submicron particles which are much more difficult to collect than are larger particles.
Consequently, there is a conflict in incinerator design: while operation at lower temperatures can minimize the vaporization of certain heavy metals, such operation would not completely destroy the toxic organic materials. Increasing the temperature can provide complete destruction of these organic materials, but makes the problem of heavy metals vaporization worse.
It is also important to note that in most cases the material being incinerated is a relatively low BTU fuel, so that some auxiliary high BTU fuel, such as gas, oil, or coal, must be used to produce a suitable operating temperature. Hence, increasing the operating temperature of the incineration process has the further disadvantage of greatly increasing the amount of auxiliary fuel which must be used, thereby increasing the operating cost.
From the foregoing, it will be appreciated that what is needed in the art are apparatus and methods for increasing the effectiveness of incinerating toxic organic compounds without increasing the operating temperature.
In addition, it would be a significant advancement in the art to provide apparatus and methods for increasing the effectiveness of incinerating toxic organic compounds existing at low concentrations without increasing the operating temperature.
It would be another important advancement in the art to provide apparatus and methods for decreasing the operating temperature of incineration processes thereby decreasing the problems associated with heavy metals vaporization, auxiliary fuel costs, and slagging and fouling, while maintaining acceptably high destruction levels of the toxic organic compounds in the waste.
Such apparatus and methods for incinerating toxic organic compounds are disclosed and claimed herein.